Of the aromatic compounds used in industry, benzene, toluene and xylenes are of outstanding importance on a volume basis. A mixture of those compounds, often designated BTX for convenience, is derived primarily from such aromatic naphthas as petroleum reformates and pyrolysis gasolines. The former result from processing petroleum naphthas over a catalyst such as platinum on alumina at temperatures which favor dehydrogenation of naphthenes. Pyrolysis gasolines are liquid products resulting from mild hydrogenation (to convert diolefins to olefins without hydrogenation of aromatic rings) of the naphtha fraction that is obtained in the steam cracking of hydrocarbons to manufacture ethylene, propylene, etc. Crude naphtha cuts are usually treated with a solvent highly selective for aromatics to obtain an aromatic extract consisting of the benzene and alkylated benzenes present in the aromatic naphtha.
The supply of BTX sometimes may be augmented by treating aromatic hydrocarbon mixtures of higher molecular weight than xylene, such as a heavy cut from petroleum reformates, in which a very high proportion of the alkyl carbon atom content is contained in the alkyl substituents on aromatic rings. The alkyl substituents in a typical reformate are, to a major extent, methyl groups, with some ethyl groups present together with a few propyl and butyl groups. Longer alkyl chains are present in such small amount that they can be disregarded. U.S. Pat. No. 3,945,913 issued Mar. 3, 1976 describes a process wherein C.sub.9 + reformate is catalytically converted to BTX by contact with type ZSM-5 zeolite, zeolite ZSM-12 or ZSM-21 at a temperature of about 550.degree. to about 1000.degree. F. U.S. Pat. No. 4,078,990 issued Mar. 14, 1978 describes a process for making BTX from heavy reformate that contains not more than 20 weight percent xylenes by catalytic contact with a solid, porous acidic catalyst characterized by a Constraint Index not higher than 1 at a temperature about 500.degree. to about 1000.degree. F. U.S. Pat. No. 3,948,758 issued Apr. 6, 1976 discloses catalytically processing heavy reformate from which benzene and lighter components have been removed to decrease the average weight of the aromatics. U.S. Pat. No. Re. 29,857, reissued Dec. 5, 1978, discloses ZSM-5 zeolites as useful in hydrocracking and other hydrocarbon conversion reactions. All of the foregoing patents are incorporated herein by reference as if fully set forth.
Recovery of the individual aromatic hydrocarbons from BTX is relatively simple for benzene and toluene. The C.sub.8 fraction, however, contains four isomers, some of which are not readily separated by distillation. Furthermore, not all of the isomers are as valuable commercially as the p-xylene, and it is desirable to convert some or all of the less desirable isomers to the para form.
Techniques are known for separating p-xylene by fractional crystallization with isomerization of the other two isomers for recycle in a loop to the p-xylene separation. That operation is hampered by the presence of ethyl benzene (EB). However, a widely used xylene isomerization technique, "Octafining" prevents build-up of EB in the separation-isomerization loop.
The manner of producing p-xylene by a loop including Octafining can be understood by consideration of a typical charge from reforming petroleum naphtha. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point F. Point F. Gal. ______________________________________ Ethyl benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
The C.sub.8 aromatic fractions from the above-described sources vary quite widely in composition but will usually be in the range 10 to 32 wt.% ethyl benzene with the balance, xylenes, being divided approximately 50 wt.% meta, and 25 Wt.% each of para and ortho.
Calculated thermodynanic equilibria for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % ethyl benzene 8.5 Wt. % para xylene 22.0 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethyl benzene by about 1 wt.%, ortho xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt.%.
In recent years processes utilizing zeolite catalysts have become available as alternatives to Octafining. A recent development in catalytic vapor phase isomerization is described in U.S. Pat. No. 3,856,872 to Morrison issued Dec. 24, 1974. It is there shown that use of a catalyst such as HZSM-5 in combination with a metal having hydrogenation/dehydrogenation promoting capability under essentially Octafining conditions is very efficient for isomerization of xylenes at reduced hydrogen flow as compared with Octafining. The extent of xylene loss is substantially reduced by this change of catalyst. Concurrently, the mechanism of ethyl benzene conversion is drastically changed on substitution of, e.g. NiHZSM-5, for the platinum on silica/alumina of Octafiners. The Morrison process results in conversion of ethyl benzene by transalkylation reactions including disproportionation of ethyl benzene to benzene and diethyl benzene, disproportionation and ethylation of xylene and the like producing alkyl aromatic compounds of nine or more carbon atoms (C.sub.9 +) together with benzene and toluene. Those conversion products are readily separated in the loop for recovery of p-xylene and isomerization of o- and m-xylenes. In general, loss of xylenes increases as severity of the isomerizer is increased to enhance the conversion of ethyl benzene. The entire contents of the foregoing patents are herein incorporated by reference as if fully set forth.
Although extensive advances have been made in processes for augmenting the supply of BTX, and in improved processes for isomerizing xylenes, significant process inefficiencies are still encountered because by-products are created in the loop. These include by-products both lighter and heavier than C.sub.8. U.S. Pat. No. 4,101,597 discloses an improved process for recovering p-xylene wherein the by-product C.sub.9 + fraction is converted to BTX and recycled to the p-xylene recovery unit. U.S. Pat. No. 4,100,214 issued July 11, 1978 discloses a vapor phase process for xylene isomerization wherein a limited amount of C.sub.9 + recycle together with toluene is added as diluent to the frest xylenes feed.